How do you specify exact colors for GMK to use? Is there a standard like Pantone or CMYK-type values? The reason I ask is because I want GMK to make some Teal-green modifiers but I don't know how to specify that in plastic coloring.

That's not possible. 8 bit colors will always look different in comparisson to real plastic colors. I think also its impossible to match a color 100%, but you can match it very close. Real life colors are dependent on light conditions and material properties. By looking at an object nobody can tell its real color. I dont know if there is kind of technique to meassure reallife colors.